1. Field of the Invention
The present invention relates to a scale for a photoelectric encoder, an encoder, and a method of forming a scale.
2. Description of Related Art
Several methods are known of forming a scale for a photoelectric encoder. One of the methods of forming the scale is a method of etching SUS (Special Use Stainless Steel), which serves as a substrate of the scale (a scale having a structure formed with this method is referred to hereafter as an etched scale structure).
FIG. 6 illustrates a partial cross-sectional view of a scale 100 having the etched scale structure. In order to facilitate understanding, cross-hatching of the substrate is omitted in the cross-sectional view. In the etched scale structure, a required contrast for the scale is obtained by a polished surface 102 of an SUS substrate 101 serving as a reflection surface and an etched surface 103, which is a portion where the SUS has been etched, serving as a low-reflection surface.
In addition, Japanese Patent Laid-open Publication No. 2009-264923 includes a method of forming a scale in which a film of a material different from that of the substrate is formed on a surface of the substrate. FIG. 7 illustrates a partial cross-sectional view of a scale 200 in which a light reflection layer is formed by Cr on a DLC film. The scale 200 includes a light absorption layer formed by a DLC (Diamond-like Carbon) layer 202 on a surface of an SUS substrate 201, and a light reflection layer 203 formed by Cr on a DLC layer and having a reflectance higher than that of the DLC layer.
The etched scale structure can be fondled at a low cost; however, an etching resist (photoresist) forming graduated marks on the SUS surface, i.e., used in a lithography technique, has poor adhesion to SUS and quality of the graduated marks is difficult to maintain.
FIG. 8 illustrates a partially expanded view of the scale having the etched scale structure. As shown in FIG. 8, in the etched scale structure, so-called undercutting may occur during etching and the quality of the graduated marks varies strikingly by location. Thus, particularly when used on a long article, the etched scale structure has been limited, such as having an accommodatable length of the scale become shorter and having low yield rates.
In addition, in the technology disclosed in Japanese Patent Laid-open Publication No. 2009-264923, a DLC film must be deposited on the surface of the substrate of the scale, which may become expensive.